1. Field of the Invention
The present invention generally relates to cutting elements for subterranean drill bits. More specifically, the present invention is directed to a novel cutting element which both serves to stabilize the bit as well as enhance bit wear life.
2. Description of the Prior Art
Diamond cutters have traditionally been employed as the cutting or wear portions of drilling and boring tools. Known applications for such cutters include the mining, construction, oil and gas exploration and oil and gas production industries. An important category of tools employing diamond cutters are those drill bits of the type used to drill oil and gas wells.
The drilling industry classifies commercially available drill bits as either roller bits or diamond bits. Roller bits are those which employ steel teeth or tungsten carbide inserts. As the name implies, diamond bits utilize either natural or synthetic diamonds on their cutting surfaces. A "fixed cutter", as that term is used both herein and in the oil and gas industries, describes drill bits that do not employ a cutting structure with moving parts, e.g. a rolling cone bit.
The International Association of Drilling Contractors (IADC) Drill Bit Subcommittee has officially adopted standardized fixed terminology for the various categories of cutters. The fixed cutter categories identified by IADC include polycrystalline diamond compact (pdc), thermally stable polycrystalline(tsp), natural diamond and an "other" category. Fixed cutter bits falling into the IADC "other" category do not employ a diamond material as any kind as a cutter. Commonly, the material substituted for diamond includes tungsten carbide. Throughout the following discussion, references made to "diamond" include pdc, tsp, natural diamond and other cutter materials such as tungsten carbide.
An oil field diamond bit typically includes a shank portion with a threaded connection for mating with a drilling motor or a drill string. This shank portion can include a pair of wrench flats, commonly referred to a "breaker slots", used to apply the appropriate torque to properly make-up the threaded shank. In a typical application, the distal end of the drill bit is radially enlarged to form a drilling head. The face of the drilling head is generally round, but may also define a convex spherical surface, a planar surface, a spherical concave segment or a conical surface. In any of the applications, the body includes a central bore open to the interior of the drill string. This central bore communicates with several fluid openings in the bit used to circulate fluids to the bit face. In contemporary embodiments, nozzles situated in each fluid opening control the flow of drilling fluid to the drill bit.
The drilling head is typically made from a steel or a cast "matrix" provided with polycrystalline diamond cutters. Prior art steel bodied bits are machined from steel and typically have cutters that are press-fit or brazed into pockets provided in the bit face. Steel head bits are conventionally manufactured by machining steel to a desired geometry from a steel bar, casting, or forging. The cutter pockets and nozzle bores in the steel head are obtained through a series of standard turning and milling operations. Cutters are typically mounted on the bit by brazing them directly into a pocket. Alternatively, the cutters are brazed to a mounting system and pressed into a stud hole, or, still alternatively, brazed into a mating pocket.
Matrix head bits are conventionally manufactured by casting the matrix material in a mold around a steel core. This mold is configured to give a bit of the desired shape and is typically fabricated from graphite by machining a negative of the desired bit profile. Cutter pockets are then milled into the interior of the mold to proper contours and dressed to define the position and angle of the cutters. The internal fluid passageways in the bit are formed by positioning a temporary displacement material within the interior of the mold which is subsequently removed. A steel core is then inserted into the interior of the mold to act as a ductile center to which the matrix materials adhere during the cooling stage. The tungsten carbide powders, binders and flux are then added to the mold around the steel core. Such matrices can, for example, be formed of a copper-nickel alloy containing powdered tungsten carbide. Matrices of this type are commercially available to the drilling industry from, for example, Kennametal, Inc.
After firing the mold assembly in a furnace, the bit is removed from the mold after which time the cutters are mounted on the bit face in the preformed pockets. The cutters are typically formed from polycrystalline diamond compact (pdc) or thermally stable polycrystalline (tsp) diamond. PDC cutters are brazed within an opening provided in the matrix backing while tsp cutters are cast within pockets provided in the matrix backing.
Cutters used in the above categories of drill bits are available from several commercial sources and are generally formed by sintering a polycrystalline diamond layer to a tungsten carbide substrate. Such cutters are commercially available to the drilling industry from General Electric Company under the "STRATAPAX" trademark. Commercially available cutters are typically cylindrical and define planar cutting faces.
There are three basic styles of prior art cutter mounting systems. A first style is a polycrystalline diamond compact with a tungsten carbide stud pressed into a hole in the bit face where the pdc is brazed to a the stud. The stud is typically available in a variety of styles including "flat top" and "round top" configurations. The assembly of stud and pdc is force fitted into a hole in a steel bit face.
A second style of mounting system is a brazed attachment of the cutter into a pocket in a tungsten carbide matrix. In this style, a backing is formed of a tungsten carbide matrix where the geometry of the backing is controlled by the shape of the mold. In a third style, a high temperature braze joint is made between the pdc and a tungsten carbide carrier. In this prior art style, the assembly is brazed into a mating pocket with low temperature braze joint.
The pdc carrier typically features a solid blocky mass positioned behind the cutter without the presence of any void areas. Likewise, in the mechanical or brazed attachment system a solid blocky mass of cast tungsten carbide is utilized behind the cutter to provide sufficient mechanical strength. This mass is positioned with one flat side against the back of the cutter with the second flat side positioned toward the bit face. This configuration causes the rounded edge to become the exposed top rear of the pocket mass.
The forward or cutting portion of each cutter mounting system is designed to provide sufficient cutter attachment and retention. The rearward or attachment portion of each system behind the cutter must provide mechanical strength sufficient to withstand the forces exerted during the drilling operation. An essential requirement of any style is that the rearward portion of the mounting system not unduly flex, break or erode.
The cutting action in prior art bits is primarily performed by the outer semi-circular portion of the cutters. As the drill bit is rotated and downwardly advanced by the drill string, the cutting edges of the cutters will cut a helical groove of a generally semicircular cross-sectional configuration into the face of the formation. When drilling well bores in subsurface formations it often happens that the drill bit passes readily through a comparatively soft formation and strikes a significantly harder formation. In such an instance, rarely do all of the cutters on a conventional drill bit strike this harder formation at the same time. A substantial impact force is therefore incurred by the one or two cutters that initially strike the harder formation. The end result is high impact load on the cutters of the drill bit. Moreover, substantial wear or even destruction of the cutters initially striking the harder formation lessens the drill bit life.
Prior art drill bits also prone to premature wear as a result of vibration. This problem is particularly acute when the well bore is drilled at a substantial angle to the vertical, such as in the recently popular horizontal drilling practice. In these instances, the drill bit and the adjacent drill string are subjected to the downward force of gravity and a sporadic weight on bit. These conditions produce unbalanced loading of the cutting structure, resulting in radial vibration.
Prior investigations of the effects of the vibration on a drilling bit have developed the phraseology "bit whirl" to describe this phenomena.
A number of disadvantages are associated with conventional cutter mounting systems. First, as the cutter wears the bearing area of the bit face on the hole bottom substantially increases. This causes an increasing amount of heat to be created, which is then conducted through the cutter mounting system. Such excessive heat is detrimental to pdc cutters.
Second, the progressively increasing wear flat area decreases product performance. Termination of the bit run occurs due to excessive torque, excessive bit weight requirements, poor penetration rate, or poor cutter retention.
Third, because of the wear characteristics and associated limitations of prior art cutter mounting systems, used bits are frequently returned from the field with greater than 50% of the original diamond material remaining on the bit. Such waste unnecessarily enhances operating costs.
Finally, prior art cutter systems have no method for damping vibration experienced as a result of drilling conditions. Such bit vibration causes cutter breakage, excessive drill string torque, and consequently, less economical drilling operations.